1. Field of the Invention
The present invention relates to the field of data filtering and, in particular, in certain preferred embodiments to the field of deblocking filters in video decoders.
2. Description of the Prior Art
In the field of video decoders, for example, in MPEG decoders, it is known to use deblocking filters to improve the picture. This is particularly important in situations where there is a low bit rate, as for example, in the transmission of video data to a mobile telephone.
Deblocking filters consist of some decision logic, which is used to decide where the boundary of a block is to be found, i.e. to detect a horizontal and/or vertical line and then to decide whether the line is part of the picture or is merely the boundary of a block and as such should be removed. This decision logic is then followed by a low-pass filter that can be applied across the block boundaries.
An example of a deblocking filter which is used in the MPEG-4 standard is given below. The matrix notation of this filter is shown in FIG. 1.
The impulse response of the filter is:(1,1,2,2,4,2,2,1,1)/16
When applied the filter operates on an input array of ten pixels (x0 to x9), and updates the middle eight pixels (y1 to y8). Whenever the filter would access pixels outside the input data, the value of the appropriate boundary pixel is substituted. A matrix notation of the filter is shown in FIG. 1. FIG. 1 shows the matrix notation of a low pass filter that acts as a deblocking filter in an MPEG decoder.
One drawback of applying this filter to the decoded data is that it is expensive to do, and in some implementations requires twice the number of cycles as the MPEG-4 decoder itself.